President Donald Trump didn't mention a price tag Monday for his lofty plan to send Americans to the moon and Mars, or say how he would pay for it — but advocates of a reinvigorated space program hailed it as a significant step nonetheless.

“I think he sees space as a symbolic area of American leadership," said John Logsdon, founder of the Space Policy Institute at George Washington University, who witnessed the 1969 launch of Apollo 11, which first took Americans to the moon. "To use a cliché, a way to make America great again."

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Unlike with the space race of the 1960s, the program Trump outlined Monday would not be entirely a NASA-run operation. Instead, the agency would enlist international partners and a new class of billionaire-run private space juggernauts to help pull it off.

The goal would be to return to the moon, then journey on to Mars — though Trump left it unclear whether the U.S. would rely primarily on humans or robots to build a lunar outpost for deep space exploration.

“This time we will not only plant our flag and leave our footprint, we will establish a foundation for an eventual mission to Mars and perhaps someday to many worlds beyond,” Trump said Monday in a signing ceremony in the Roosevelt Room at the White House, flanked by dignitaries including Apollo 11 veteran Buzz Aldrin and former Sen. Jack Schmitt (R-N.M.) of Apollo 17, who brought with him Lunar Sample 70215, a piece of basaltic lava rock retrieved in 1972.

The policy also marks a shift from the Obama administration's plan to bypass the moon and focus on reaching asteroids to eventually explore Mars.

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"I think it is a good sign," said Lori Garver, who was deputy NASA administrator under former President Barack Obama, crediting influential space enthusiasts close to Trump. "I think with the combination of Newt Gingrich and Pence himself they were able to get it through."

"People have pushed for eight years for this moment," added Lauren Worley, who served as chief NASA spokesperson during the Obama years.

But some the biggest enthusiasts for returning to the moon have questions about Trump's plan.

"Great visions have to be backed up with the funding to make in reality," said Bob Richards, CEO of Moon Express, which hopes to play a role in the effort and holds the only government license to land a probe on the moon next year. "It says humans on the moon but it doesn't say how. We look forward to a future that was painted today of a much larger and broader program."

How such a program will be designed and how it will be paid for remain key unanswered questions facing NASA and Congress, current and former space officials agreed.

But the White House made clear that the president sees revitalizing the space program as a key ingredient to returning American leadership to the global scene, including economically.

In a fact sheet released Monday, the administration bemoaned what it sees as a steady slide in America’s space prominence since a dozen Americans walked on the moon between 1969 and 1972.

"We are no longer the undisputed leader in human space exploration," it says, adding that "after the Space Shuttle was retired, the United States has been forced to rely on Russian rockets, at the cost of $70 million per seat."

"In the coming years the United States will launch astronauts on an American-made rocket and crew system, the Space Launch System and Orion crew vehicle, and multiple American companies will provide the Pentagon with American engines and rockets to launch national security payloads," the White House document pledges.

Those programs have been in the works through successive presidential administrations.

Enlisting the help of non-traditional space companies such as Elon Musk's SpaceX and Jeff Bezos' Blue Origin is also considered key to a broader strategy to tap into an emerging private economy.

"America will harness the power of private industry," the fact sheet declares. “By refocusing our space program on feasible goals, President Trump will create incentives for private industry that spur 21st century space capabilities."

Space enthusiasts and industry leaders applauded the call for cooperation between the space agency and the growing private space sector that is driving much of space innovation — not just traditional defense and aerospace contractors such as Lockheed Martin, which is building the Orion spacecraft.

Eric Stallmer, president of the Commercial Spaceflight Federation, who was on hand at the signing ceremony, said he sees the announcement as strong signal that private players will play a prominent role.

“I think they want to have the best and brightest and most innovative ideas,” he said. “If we go onto the moon and beyond, it’s not going to be done by the traditional ways that the government has done things the past 50 years.”

More traditional space technology companies and their representatives also expressed optimism.

"On the 45th anniversary of the Apollo 17 mission, it’s particularly appropriate to note the strong, sustained partnership between government and industry that enables our drive to explore the cosmos," David Melcher, the CEO of the Aerospace Industries Association, said in a statement.

Lockheed Martin, which is building the Orion Multi-Purpose Crew Vehicle, said in a statement that going to the moon with today’s advanced technology “would further our understanding of the Moon’s history and resources."

"And it will build a strong foundation that will not only accelerate the U.S. to Mars and beyond, it will foster a thriving new space economy that will create jobs and drive innovation here on Earth," it added.

NASA boosters also hailed the move.

"By signing this directive, President Trump has again shown that, under his administration, America will be a leader in space exploration," saidHouse Science Chairman Lamar Smith (R-Texas).

Pence, in a statement, called space the "next great American frontier" and said "it is our duty — and our destiny — to settle that frontier with American leadership, courage and values."

But how the moon program will take shape remains to be seen, said Andrew Aldrin, who runs the Florida Institute of Technology's Buzz Aldrin Space Institute, which is named for his father.

NASA may be building spacecraft that could get to the moon but is not building a vehicle to actually land there.

"Now that there is a much clearer destination and so now we have to build landers," Aldrin said. "You have to have a plan and have to work on the real details of it."

Another pathway may be to build a so-called gateway that orbits the moon but relies primarily on robots working on the surface.

"Is the next big project going to be an orbiting outpost or gateway or is it going to be a lander?" Aldrin asked. "I assume the purpose of the policy is to have people on the moon."

Much will come down to Trump's pick to run NASA, Rep. Jim Bridenstine (R-Okla.), who is awaiting Senate confirmation.

"I have long been a proponent of returning to the moon, particularly as a proving ground for missions deeper into space," Bridenstine said earlier this year.

In the meantime, NASA's acting administrator, Robert Lightfoot, pledged in a statement Monday that the agency "will engage the best and brightest across government and private industry and our partners across the world to reach new milestones in human achievement."

Added Garver, the former Obama NASA official who is now general manager of the Air Line Pilots Association: "It remains to be seen what of all that will work. But at this point it is a good sign."